The present invention relates to electrical fuselinks and, more particularly, to fuselinks having improved surge-resistant characteristics, for example, a 1 mS delay factor&gt;200.
The delay factor or D.F. is a measure of a fuselink's surge resistance and is defined by the ratio of I.sub.s, /I.sub.f, where I.sub.s is the current required to blow the fuse in a short specified time (1-10 mS), and I.sub.f is the minimum fusing current, that is, the least current which will ultimately blow the fuse if allowed sufficient time.
It has been discovered that one parameter which significantly influences the D.F. of a fuselink is the heat loss from the fuse element. The greater the heat loss, the less is the delay factor. In a conventional cartridge fuselink, for example, some heat is conducted axially along the fuse element to the end caps and a small amount is radiated from the surface of the fuse element but, in an air-filled fuselink, most of the heat loss is by convection to the surrounding ceramic or glass barrel. For example, an increase of 2.7:1 in the D.F. of a 20.times.5 mm cartridge fuselink could be expected if it were practicable to reduce the heat loss by evacuating the air-space within the insulating barrel.
Moreover, it has been discovered experimentally that the introduction of any of the conventional solid thermal insulants into the air-space within the insulating barrel of a cartridge fuselink (e.g. a 20.times.5 mm fuselink) has the surprising detrimental effect of increasing and not decreasing the heat loss. The thermal conductivity of the solid material with its entrapped air is greater than that of free air in a fuselink of this size. The materials evaluated included fibreglass, polystyrene foam and vermiculite. Of course, the provision of a vacuum or reduced air pressure within the space in the insulating barrel would provide for reduced heat loss in relation to that achieved with free air but such a provision is not generally a practical or economical proposition for cartridge fuselinks.